package everyday.apr;

/**
 * 357. 统计各位数字都不同的数字个数
 */
public class Day11 {
    public static void main(String[] args) {
        System.out.println(countNumbersWithUniqueDigits(2));
        System.out.println(countNumbersWithUniqueDigits(0));
    }

    /**
     * 数学 -- 排列组合
     */
    public static int countNumbersWithUniqueDigits(int n) {
        if (n == 0) return 1;
        if (n == 1) return 10;

        int res = 10, cur = 9;
        for (int i = 0; i < n - 1; i++) {
            cur *= 9 - i;
            res += cur;
        }

        return res;
    }
}
